In various modern electronic or photonic applications, single-crystalline thin film materials are widely utilized in order to enable manufacturing of functional electronic or photonic devices. For example, thin films of electro-optic materials are needed to make optical modulators, electro-optic devices, acousto-optic devices, and nonlinear optical devices. Bonded thin film of semiconductors are particularly useful for fabrication of high-speed or high-power electronic devices. Furthermore, high resolution x-ray detectors often require bonded thin films of scintillator crystals. A wafer-bonding step is necessary to make the thin film layer, which is particularly useful in manufacturing of various high-speed, high-power, x-ray, and/or photonic devices.
Several conventional methods exist for semiconductor and dielectric substrates wafer bonding involving direct or indirect bonding. In indirect wafer bonding techniques, an intermediate layer, such as a glue or some adhesive, is used to attach two wafers. On the other hand, in direct wafer bonding techniques, no adhesive or intermediate layer is needed. For many electronic and photonic applications, indirect wafer bonding methods are not desirable due to high-temperature processing steps during fabrication of devices that are not compatible with adhesive bonding. Furthermore, adhesive bonding methods are not reliable and may not be transparent to optical wavelengths, thus making the conventional indirect wafer bonding techniques generally undesirable for optoelectronics manufacturing.
Direct wafer bonding does not have an adhesive or intermediate layer between the wafers. The direct wafer bonding techniques can be divided to high temperature, low temperature, or room temperature methods. In high temperature methods, wafers need to be heated to a high temperature to achieve wafer bonding. This method works for bonding similar materials or materials that have exact same coefficient of thermal expansion (CTE). If the materials have different CTE the bonding will cause significant stress in the substrates and is not practical to use this method for bonding dissimilar materials. Similarly, low temperature wafer bonding is not appropriate for bonding of materials with different CTE.
In recent years, room temperature wafer direct bonding has been introduced for industrial applications. This new method uses an ultra-high vacuum (UHV) environment. In general, there are two methods for room temperature wafer bonding. Surface Activated Bonding (SAB) and Atomic Diffusion Bonding (ADB) are two methods that have been developed to achieve wafer bonding process (T. Shimatsu, M Uomoto, “Atomic diffusion bonding of wafers”, J. Vac. Sci. Tehnol. B 28(4), pp 706-714, 2010, E. Higurashi, T Suga, “Review of Low-Temperature Bonding Technologies and Their Application in Optoelectronic Devices”, Electronic and Communications in Japan, Vol. 99, No 3, pp 63-71, 2016).
In SAB, the surface of the wafers is plasma-treated in a UHV chamber where the native oxide layer on a surface is removed and dangling bonds are formed on the surface. These dangling bonds then allow wafer bonding to be formed in the UHV chamber to achieve bonded substrates. The activated surfaces are brought into contact in UHV chamber where the dangling bond can form a covalent bond between two substrates. The covalent bond is very strong. Hence the bond strength for these bonded substrates is very strong. The SAB method is more useful for bonding of metals or semiconductors.
For lithium niobate or other single-crystalline oxide materials, the SAB method may not work well because no dangling bond can be formed on the surface by the plasma treatment when the material for targeted bonding is an oxide material. For bonding of single-crystalline oxide materials, ADB method is employed in which a mono-layer of metals is deposited on the surface of the crystal in UHV chamber. This monolayer metal need to be deposited in a UHV chamber in order to prevent oxidation of the metal. The metal creates dangling bonds on the wafer surface where the dangling bonds can interact and form a covalent wafer bonding similar to the covalent bonding formed by the SAB method.
The ADB might be compared to an adhesive-based bonding method where the mono-layer metallic layer is the adhesive between the two substrates. However, the thickness of the deposited metal layer is a mono-layer or a sub-monolayer. The small amount of deposited material can easily be oxidized or diffused into the substrate, and hence it will not act as a metallic layer. Because the thickness of this layer is very thin and is often less than a mono layer, it will not interfere with electronic or photonic properties of devices fabricated on such substrates. Because the amount of deposited material is a sub-monolayer, it has insignificant or no effect on device performance. Therefore, the ADB method can also be classified as a direct bonding method.
For optical waveguide and opto-electronic device fabrication and manufacturing, a wafer bonding is needed between optical single-crystalline materials and a substrate. The SAB method does not work well in this case because the dangling bond does not exist in an oxide material. The ADB method is generally more suitable for manufacturing applications that require bonding of single crystalline oxides. A thin layer of oxide material will then be formed after bonding a single-crystalline material to a second substrate using crystal ion slicing or thinning. The mono-layer or the sub-mono layer metal layer deposited during bonding may still cause problems because a mono-layer material can still absorb light, which is not desirable in an optical waveguide or opto-electronic device applications. However, an annealing step can be used to agitate the mono-layer metal to be oxidized and diffused into the single-crystalline material, which ensures that a free electron absorption caused by metallic state is eliminated and a bonded thin-film layer is transparent to optical signals.
It may be desirable to devise a novel apparatus and a corresponding manufacturing process to accommodate wafer-bonding steps and fabricate thin films of various electronic or photonic materials. In particular, it may be desirable to devise a novel polished surface-bonding apparatus that provides bonding of two different single-crystalline or amorphous materials at room temperature using ADB or SAB methods. Furthermore, it may also be desirable to devise the polished surface-bonding apparatus to utilize uniquely-structured hinges to place wafers inside an ultra-high vacuum (UHV) environment and a plasma source for achieving surface activation.
In addition, it may also be desirable to devise a novel polished surface-bonding apparatus that exhibits a streamlined and low cost wafer-bonding procedure with a high bonding throughput. Moreover, it may also be desirable to devise a novel device manufacturing method to produce low-loss optical waveguide devices with high productivity by utilizing ADB and annealing techniques.